Clinical - CMT1A is a demyleinating peripheral neuropathy characterised by distal muscle weakness and atrophy associated with mild/moderate glove & stocking sensory loss, depressed reflexes and pes cavus. CMT1A follows an autosomal dominant pattern of inheritance with age-dependent penetrance and variable expressivity. It usually presents between the ages of 5-15 years with difficulty walking due to problems picking up the feet ('foot drop'). Gradual loss of muscle bulk in the lower calves leads to weakness wth dosiflexion and eversion of the foot. Weakness of the hands may occur later and is rarely symptomatic before adult life. The weakness is very slowly progressive. Affected individuals may also have a loss of sensation in the hands and feet.
Sometimes CMT1A presents considerably later, even into middle age and a small percentage (~10%) of individuals remain largely asymptomatic in adult life, but with changes detectable on careful clinical assessment and/or nerve conduction studies.
The CMT1A duplication incorporates the PMP22 gene. Peripheral myelin protein 22 is a major component of myelin expressed in the compact portion of essentially all myelinated fibers in the peripheral nervous system and is produced predominantly by Schwann cells. In CMT1A, nerve conduction velocities (NCV's) are slow, typically 10-30 m/s.
Size of duplication - The 1.5 Mb duplication at 17p11.2, which defines CMT1A is a recurrent duplication flanked by low copy repeats.
Origin of duplication - The duplication arises from unequal crossing over of homologous chromosomes at regions of low copy repeats flanking the duplicated region. Deletion of the same region causes the milder phenotype Hereditary Neuropathy with Liability to Pressure Palsies (HNPP). In ~2/3rds of individuals the duplication is inherited; it arises de novo in the remaining 1/3rd.
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